Define smooth muscle and its primary locations in the body.

Smooth muscle is defined as non-striated, involuntary muscle found primarily in the walls of hollow organs, such as the intestines, bladder, blood vessels, and the uterus. This type of muscle is responsible for involuntary movements, such as peristalsis in the digestive tract and the regulation of blood flow through the contraction and relaxation of blood vessel walls.

The classification of smooth muscle as non-striated is key, as it differs from the striped appearance of skeletal and cardiac muscle. The involuntary nature of smooth muscle means it operates autonomously, controlled by the autonomic nervous system rather than conscious effort. This allows for critical involuntary processes to occur without the need for conscious control, ensuring the proper functioning of the body's organs.

In contrast to this, skeletal muscle, which is striated, is primarily associated with body movement and is found in limbs and the torso. Cardiac muscle, also striated but found exclusively in the heart, is similarly involuntary but distinct in its function as it pumps blood throughout the body. Voluntary muscle pertains to skeletal muscle, highlighting the differences in how various muscle types are activated and utilized within the body.